1. Field of the Invention
The invention concerns the field of magnetic resonance (MR) imaging. In particular, the invention concerns methods and devices for MR imaging in which a magnetization of an examination subject is specifically prepared.
2. Description of the Prior Art
MR techniques have become widespread over time and have numerous applications in medical technology. In one class of methods for MR imaging, the magnetization of hydrogen nuclei spins is globally or locally affected in order to subsequently acquire MR data with a readout sequence. Examples of such methods contain the targeted saturation of hydrogen nuclei spins of free water to increase contrast.
A technique known as CEST (Chemical Exchange-dependent Saturation Transfer) imaging is a relatively recent method for MR imaging. This is described for an exemplary application in W. Ling et al., “Assessment of glycosaminoglycan concentration in vivo by chemical exchange-dependent saturation transfer (gagCEST)”, Proc Natl Acad Sci USA 2008; 105(7):2266-2270. In general, in CEST imaging a magnetization of hydrogen nuclei spins is generated from molecules or molecule groups selected depending on the application, the hydrogen atoms of which are transferred to water molecules via chemical exchange with said water molecules. For preparation of the magnetization, alternating magnetic fields can be used whose frequency corresponds essentially to the resonance frequency of the hydrogen nuclei spins in the selected molecules and has a frequency shift relative to resonance frequency of the hydrogen nuclei spins of free water. A readout sequence can subsequently be implemented with which the magnetization of hydrogen nuclei spins in water molecules is read out. Via the chemical exchange, the prepared magnetization of the nuclear spins of the molecules of interest is transferred to water and can be detected using the acquired MR data. CEST imaging can be used for different purposes, for example to depict cartilage damage, to determine the pH value of tissue, for spatially resolved depiction of protein concentrations in the brain (the —NH groups of which protein concentrations can be detected with CEST imaging) or for in vivo imaging of gene expression.
For example, in CEST methods MR data can be repeatedly, sequentially acquired, wherein the examination subject is respectively exposed to an alternating field with a frequency to prepare the magnetization. This process is repeated for alternating fields with different frequencies. For example, in the article by W. Ling et al. cited in the preceding alternating fields with two frequencies are used that are symmetrical to the resonance frequency of hydrogen nuclei spins in free water. Deviations of the basic magnetic field from a known field strength (that, for example, can be caused by an inhomogeneity of the basic field) can lead to the situation that the quality of the image data determined with the CEST imaging is reduced. In the extreme case, for example, basic field inhomogeneities can lead to the situation that the frequency of the alternating fields is so far removed from the actual possible, locally varying resonance frequency of the hydrogen nuclei spins that reasonable conclusions with regard to the presence or absence of the molecules that should be depicted with spatial resolution can no longer be drawn from the acquired MR data.